Ankylosing spondylitis is a chronic inflammatory disease of the axial skeleton with
variable involvement of peripheral joints and nonarticular structures. AS is a form
of spondyloarthritis, a chronic, inflammatory arthritis and autoimmune disease. It
mainly affects joints in the spine and the sacroiliac joint in the pelvis, and can
cause eventual fusion of the spine.

It is a member of the group of the spondyloarthropathies with a strong genetic predisposition.
Complete fusion results in a complete rigidity of the spine, a condition known as
"bamboo spine".